Add () in the method call

function method2()
    {
        $myvar = firstclass::method1();
        return $myvar;
    }

OR

function method2()
        {
            $firstObj= new firstclass();
            $myvar = $firstObj->method1();
            return $myvar;
        }

class firstclass
{
    public static function method1()
    {
       return "foo";
    }
}
class secondclass
{
    public function method2()
    {
        $myvar = firstclass::method1();
        return $myvar;
    }
}

Your source code is not a good example of object oriented programming at all.

You SHOULD always pass firstclass into a secondclass using for example constructor. Then you can call first class from inside the secondclass.

Object injection

class firstclass
{
    function method1()
    {
        return "foo";
    }
}

class secondclass
{
    /**
     * @var firstclass
     */
    private $firstClass;

    /**
     * @param firstclass $firstClass
     */
    public function __construct(firstclass $firstClass)
    {
        $this->firstClass = $firstClass;
    }

    function method2()
    {
        return $this->firstClass->method1();
    }
}

print_r((new secondclass(new firstclass))->method2());

Callable injection

If you don't want use the whole instance of firstclass inject callable inside the secondclass and then call it.

Calling method instance

class firstclass
{
    function method1()
    {
        return "foo";
    }
}

class secondclass
{
    /**
     * @var callable
     */
    private $method1;

    /**
     * @param callable $method1
     */
    public function __construct(callable $method1)
    {
        $this->method1 = $method1;
    }

    function method2()
    {
        return call_user_func($this->method1);
    }
}

print_r((new secondclass(array(new firstclass, 'method1')))->method2());

Calling static method

class firstclass
{
    static function method1()
    {
        return "foo";
    }
}

print_r((new secondclass('firstclass::method1'))->method2());